Methods involving deposition of a metal oxide layer via liquid phase decomposition (hydrolysis) of a corresponding salt (i.e. sulfate or halide) are known per se and have been used to form luster, or pearlescent pigments which have translucent, non-reflective mica core materials. However, such methods, described for example in U.S. Pat. No. 3,087,827 and U.S. Pat. No. 5,733,371, have not been considered suitable for forming effect pigments with reflective metallic cores in the highly acid (pH of less than 4), aqueous solutions required by such processes. U.S. Pat. No. 6,369,147 discloses a process that solves the foregoing problem by selecting certain metal cores and optionally treating them in such a way that they are rendered more corrosion resistant.
Use of microwave energy for the deposition of metal oxide films onto glass and indium tin oxide coated glass plates used for LED devices is known and disclosed in numerous journal articles such as E. Vigil, L. Saadoun, Thin Solid Films 2000, 365, pp 12-18 and E. Vigil, L. Saadoun, J. Materials Science Letters 1999, 18 pp 1067-1069. Good adhesion was obtained only on indium tin oxide coated glass plates, which the authors suggested was due to some electron donation ability of the indium tin oxide coating (see Vigil, E.; Ayllón, J. A.; Peiró, A. M.; Rodriguez-Clemente, R.; Doménech, X.; Peral, J. Langmuir 2001, 17, 891).
The bulk precipitation of metal oxide particles by microwave irradiation is well known. For examples of bulk precipitation oxides using microwave deposition, see (1) Lerner, E.; Sarig, S.; Azoury, R., Journal of Materials Science: Materials in Medicine 1991, 2, 138 (2) Daichuan, D.; Pinjie, H.; Shushan, D. Materials Research Bulletin, 1995, 30, 537 (3) Leonelli, C. et al., Microwaves: Theory and Applications in Materials Processing 2001, 111, 321, (4) Girnus, I. et al., Zeolites 1995, 15, 33, (5) Rodriguez-Clemente, R. et al., Journal of Crystal Growth 1996, 169, 339 and (6) Daichuan, D.; Pinjie, H.; Shushan, D. Materials Research Bulletin, 1995, 30, 531.
Surprisingly, applicants have found that use of the microwave deposition process of the present invention allows for a process for the deposition of uniform, semi-transparent or transparent, thin films of metal oxides on cores of uniform thickness, which thickness can be adjusted based upon the mass ratio of the organic substrate material to the metal oxide (mass of metal oxide precursor material), allowing for the preparation of thin films of metal oxides of a variety of thicknesses depending upon the desired effect without precipitation of the metal oxide. When the metal oxide layer is made with liquid phase deposition, and conventional heating is applied, energy is transferred from the surface to the inorganic bulk mixture and eventually to the substrate material. With microwave treatment, energy is focused on the substrate material due to the better absorbance of the microwave energy by the substrate than the bulk mixture. This will make the substrate the reaction center, which allows the reaction to take place with higher probability at the surface of the substrate. Reaction at the surface results in better adhesion of the coating layer and significantly less bulk precipitation. The good surface adhesion, easy adjustment of reaction conditions to change the thickness or composition of the coating, as well as minimal deposition into the bulk media, are significant advantages of the instant invention.
Accordingly, it is an object of the invention to provide a process of using microwave deposition of metal oxide layers onto an organic substrate as defined hereinafter. The coated organic material can exhibit an optical goniochromatic effect. Or otherwise the organic material can be dissolved to yield free metal oxide or mixed metal oxides that exhibit an optical goniochromatic effect.